linux-kernel - Re: [RFC PATCH devicetree] of: property: mark "interrupts" as optional for fw

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Message-ID: <CAMuHMdUso+j5K8VYxTofg7Pw4jdH2xMMGBXGwBxpzRiQWX+ZNA@mail.gmail.com>
Date:   Fri, 20 May 2022 09:02:05 +0200
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Saravana Kannan <saravanak@...gle.com>
Cc:     Vladimir Oltean <vladimir.oltean@....com>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Robin Murphy <robin.murphy@....com>,
        Geert Uytterhoeven <geert+renesas@...der.be>,
        Rob Herring <robh+dt@...nel.org>,
        Frank Rowand <frowand.list@...il.com>,
        John Stultz <john.stultz@...aro.org>,
        Alvin ┼áipraga <alsi@...g-olufsen.dk>,
        Florian Fainelli <f.fainelli@...il.com>,
        Andrew Lunn <andrew@...n.ch>,
        Russell King <rmk+kernel@...linux.org.uk>,
        Heiner Kallweit <hkallweit1@...il.com>,
        Android Kernel Team <kernel-team@...roid.com>
Subject: Re: [RFC PATCH devicetree] of: property: mark "interrupts" as
 optional for fw_devlink

Hi Saravana,

On Thu, May 19, 2022 at 10:00 PM Saravana Kannan <saravanak@...gle.com> wrote:
> There's still work to be done that might make this easier/cleaner in the future:
> 1. Adding a DT property that explicitly marks device A as not
> dependent on B (Rob was already open to this -- with additional
> context I don't want to type up here).
> 2. Adding kernel command line options that might allow people to say
> stuff like "Device A doesn't depend on Device B independent of what DT
> might say".

There are clearly cases where the hardware defines if the property
is optional or required, and a DT property would work.  However, in
general relaxing such dependencies involves a complex mix of hardware
capabilities, driver support, and system policies.
Examples:
  - A hardware block may support both DMA and PIO, or can require DMA,
  - DMA-capable devices can typically work without an IOMMU,
    unless all RAM lies outside the address space addressable by
    the DMA controller,
  - A driver may fall back to PIO if DMA is not available (yet),
    but doing so may not meet the required performance target,
  - Not using the IOMMU may violate anti-tampering policies.

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds